If you want to plug the drain between the fuel tank compartment to the stern bilge, this is easily done after a quick visit to your local Ace Hardware store.
All you need are two inexpensive items:
1. a Toilet Wax ring seal, and
2. a 3 foot piece of 1x2
On my 330, my arms are not long enough to reach way down to the drain area.
So, you place a section of the toilet wax ring on the end of the 1X2 and shove the material into the drain.
Repeat as necessary to plug the drain(s).
And, with the same 1X2, you can later remove the wax plug by reversing what you did to install it.
I did this a year ago and completely isolated my fuel compartment from the aft bilge.
I however removed my wax plug in favor of draining water - which perhaps I never get in there (?).
So... I'm thinking maybe I will put the wax plug back in - for safety reasons - whatever that means...